Investigating the role of valence electrons in the magnetism of molecular crystals

Molecular crystals, which are electrically conductive and magnetic, provide valuable insight into valence electrons due to their low impurity concentrations. They have linked charge ordering to superconductivity and have contributed to the search for quantum spin liquids in which electron spins remain disordered even at extremely low temperatures.

Valence electrons with quantum properties are also expected to exhibit emergent phenomena, making these crystals essential for studying the functionality of new materials.

However, it is still unclear to what extent valence electrons in molecular crystals contribute to magnetism, and research on their quantum properties remains insufficient. To address this, the research team used light to analyze the configuration of valence electrons, building on studies of superconductors and quantum spin liquids. The results of this study are published in Physical Review B.

A molecular crystal (C₂H₅)(CH₃)₃As(Pd(C₃S₅)₂)₂ contains (Pd(C₃S₅)₂)₂ molecules at the vertices of a triangular lattice, with one valence electron “formally” assigned to each vertex. It is being The actual distribution of these electrons was determined experimentally. Synchrotron infrared light and near-infrared and visible laser light were used to illuminate the crystal and induce molecular vibrations.

Analysis of the vibrational frequencies revealed details such as the location of the valence electrons, the range of mobility, and whether the intermolecular distances are constant or variable. This made it possible to study the arrangement of valence electrons within the crystal.

Initially, this material was thought to be magnetic, but it was discovered that about half of the valence electrons do not contribute to magnetism and instead form pairs. These pairs exhibited properties reminiscent of superconducting states driven by charge fluctuations.

As the temperature decreases, the number of pairs increases and eventually saturates, while the remaining electrons form an antiferromagnetic configuration. At extremely low temperatures, electrons that contribute to magnetism and non-magnetic electrons coexist, forming a fixed arrangement on a triangular lattice. This mixed state closely resembles the frozen configuration of dynamic valence electrons observed in spin-liquid candidate materials.

This study reveals the hidden potential of valence electrons that do not contribute to magnetism in revealing properties related to superconductivity, magnetoresistance, and spintronics. Furthermore, it bridges the gap between non-magnetic superconductors and magnetism-related superconductors, paving the way for future research on quantum material properties.